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Comparing ZooKeeper and Consul

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Building Service Discovery with ZooKeeper or Consul? Here's a small comparison between two popular tools.

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Comparing ZooKeeper and Consul

  1. 1. ZooKeeper vs Consul Ivan Glushkov ivan.glushkov@gmail.com @gliush v1.2, Nov 2014
  2. 2. ZooKeeper: Overview ZooKeeper is a high-performance coordination service ❖ Written in Java for distributed applications ❖ Strongly consistent (CP) ❖ Zab protocol (Paxos-like) ❖ Ensemble of servers ❖ Quorum needed (majority) ❖ Dataset must fit in memory
  3. 3. ZooKeeper: Data ❖ In-memory data ❖ Shared hierarchal namespace (FS-like) ❖ Ephemeral nodes exists while session is alive ❖ Access Control List (ACL) to each node ❖ Watches on nodes.
  4. 4. ZooKeeper: API ❖ create ❖ delete ❖ exists ❖ get data ❖ set data ❖ get children ❖ sync
  5. 5. ZooKeeper: Problems ❖ Latency-dependent ❖ No multi datacenters support (very slow writes) ❖ Client number limit (out of sockets) ❖ Designed to store data on the order of kB in size ❖ Possibly lost changes while reenabling watch ❖ Need to read after every watch callback (to sync) ❖ TCP connection != service healthy (proxy) ❖ Change zk server is a problem
  6. 6. Consul Consul is a tool for discovering and configuring services in your infrastructure ❖ Written in Go ❖ Service discovery ❖ Health checking ❖ Key/Value store ❖ Multi Datacenter support ❖ Agent/Server concept ❖ Gossip protocol for all the nodes (discovery) ❖ Consensus protocol (Raft-based) for servers ❖ 3 consistency modes ❖ Access Control List (ACL)
  7. 7. Consul: HTTP API ❖ kv - Key/Value store ❖ agent - Agent control ❖ catalog - Manages nodes and services ❖ health - Manages health checks ❖ session - Session manipulation ❖ acl - ACL creations and management ❖ event - User Events ❖ status - Consul system status
  8. 8. ZooKeeper vs Consul With strong consistency Key/Value 3 consistency modes HTTP API Service registering Config file or HTTP API TCP Connection Health Check Connection to node + Any script on node + HTTP API HTTP API Service Discovery HTTP API + DNS + Configuration templates
  9. 9. Key/Value ❖ ZooKeeper: REST API. Needs quorum, has only strong consistency ❖ Consul: REST API. 3 modes: • ‘consistency’ • ‘default’ (potentially stale values = default for zk) • ‘stale’ (read from any server, often stale values, fastest mode)
  10. 10. Service registering ❖ ZooKeeper: K/V via HTTP request to servers (ephemeral nodes) ❖ Consul: local agent registering • through HTTP request • config file
  11. 11. Health Check ❖ ZooKeeper: HTTP Connection. Might not be relevant: • proxy holds connect • node is alive, while the app hangs ❖ Consul: gossip for node aliveness, preconfigured health checks (any script), HTTP API
  12. 12. Service Discovery ❖ ZooKeeper: HTTP request to the K/V ❖ Consul: DNS or HTTP API
  13. 13. More Consul pros ❖ Consul is built on top of Serf - node discovery and orchestration tool (gossip model). It’s used for group membership, failure detection and event broadcast: • LAN-pool for membership and failure detection (distributed checks, good for large clusters) • WAN-pool for inter-datacenter consul servers communication ❖ Consul templates to update files and possibly restart applications (haproxy with backend servers)
  14. 14. Links ❖ Zk overview: http://zookeeper.apache.org/doc/trunk/zookeeperOver.html ❖ Zk experience: http://whilefalse.blogspot.ru/2012/12/building-global-highly-available. html ❖ Test zk: http://aphyr.com/posts/291-call-me-maybe-zookeeper ❖ Restart zk server problem: http://zookeeper-user.578899.n2.nabble.com/How-to-replace- a-zookeeper-server-td7472559.html ❖ Sample consul use for HAproxy configuration: https://hashicorp.com/blog/ haproxy-with-consul.html ❖ Consul templates feature: https://github.com/hashicorp/consul-template/releases ❖ Stale reads from ZooKeeper: http://stackoverflow.com/questions/5420087/ apache-zookeeper-how-do-writes-work

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